IT'S A COLD, drizzly day as I ride outbound from ­Boston, shadowing the Orange Line on the Pierre Lallement bike path. A teenage girl who has been flying along a half-block ahead of me comes to a halt at a quiet intersection across from the Roxbury Crossing T stop. The light is long. She is patient, has a sturdy rack for her book bag, a bell on her bar, and a Muslim hijab beneath her helmet. From her appearance and demeanor, I determine she must be heading to the Earn-A-Bike class at the Hub, the Jamaica Plain headquarters of Bikes Not Bombs.

When we both end our ride there, I’ll find out my guess was correct—but that I was wrong to think that Fatima, an 18-year-old immigrant from Somalia, is one of the students. She turns out to be a teacher.

This is only one of the through-the-looking-glass revelations to be had at the Hub, the nickname for the old warehouse on Amory Street where teens teach, battered bicycles become swoon-worthy, and, for three hours, four afternoons a week, city youth dedicate themselves to what must be acknowledged as one of the most socially unhelpful, style-wrecking activities a modern teen could undertake: bicycle mechanics. Rahsaan Bahati, an African-American former U.S. national criterium champion, has said that there was such a stigma attached to cycling when he attended high school in South Central LA that he hid his bike shorts for fear he’d be found out—and he at least was a racer, which has some element of danger and speed. There’s no sense in pretending that the two dozen young people who are here don’t know the schoolyard rep. But they also don’t care.

In front of the class now, Fatima starts off the afternoon with a quick game of Bike Tool Jeopardy. She wears a light blue hoodie and has a smile that is warm and serene. A year ago, the thought of applying a wrench to a bike was like a Fear Factor dare to her. Today, she stands amid three shop benches populated with various tools, as members of three teams, lined single-file, hang on her whisper-light questions so they can lunge to be the first to pick up the tool she describes: You use this to toe-in your brake pads...

After 15 minutes or so, when the third-hand tools and crank extractors and all the rest have been snatched up amid escalating smack talk and screeches, the ­students start to settle into the day’s tasks. The class is just three days shy of a graduation ceremony at which the teenagers are supposed to ride their finished bikes across this very same warehouse floor. There is work to be done. Annie, a strongly built African American girl, still seems too jazzed from the contest to get focused on her BMX, but she is the exception. Kelly, a swarthy Latino girl in a sparkly Hello Kitty top, makes so much headway in her rear triangle that ­Jesus applauds her and exclaims, “You’ve got mad skills!” A few bike stands down, an older, tall, light-haired big-sister type named Kate eyeballs her Woodstock-era frame like a 15th-century Florentine artist trying to pull the trigger on a new front door for the Duomo. She is seized with the irresistible urge to overhaul her front hub for, she says, like the fourth time. She spins a wheel and points at it. “See the woop?” she says to me.

I don’t—just one way I am out of my element. I’m a small-town guy and a mechanical fumbler, out of sync amidst the R&B background music as well as the urgent conversations about crankarms and cantilevers. During the derailleur primer, I’d sat awkwardly at the back of the room, hoping not to get called on, and wondering if I’d raised my son to be too much like me. Last summer Henry, 15, disassembled three bikes to create one fixie but ended up creating only a component-and-tool goulash that stayed spilled across the floor of our garage for weeks.

The six-week-long earn-a-bike course is defined as a chance for students to “learn bike mechanics and life skills as they refurbish their bikes.” It has been offered in one form or another since the early 1990s, and graduates number in the thousands. The students are usually 12 to 18, with limited to no experience working on bicycles. There are classes like the one I’m shadowing, reserved for girls, and children as young as six have gone through a related program to earn a bike here. One staffer remembers with something like awe the image of a first-grader working fresh grease through new bearings.

The class I am visiting is fairly typical in its demographic makeup, with youth from vocational, private, and large public schools. Most are not the sons and daughters of avid cyclists. Some were edged through the door by a parent who cared less about bikes and more about busyness. The originator of the course, a group called Bikes Not Bombs, knows the territory. It is one of Boston’s most revered cycling institutions,­ a 28-year-old not-for-profit whose mission is to use the “bicycle as a vehicle for social change.” The founding idea, unabashedly lefty, was to collect used bikes, restore them, then ship them to beleaguered and bullied developing countries like Nicaragua. Founder Carl Kurz started with two bikes he personally refurbished; an estimated 46,000 have now found foreign homes. The group continues to expand its focus and, through programs of its own and related city-run ones, they’ve helped bring hundreds of reconditioned bikes to largely low-income households. This is part of a movement that has ushered Boston into an unprecedented bike-friendly era. (In 2009, the mayor stated, “the car is no longer the king in Boston.”) Increasingly, the energy, focus, and vibe are about working a simple machine into the complicated lives of city youth. The educational precept is that knowing your way around a bike imprints a set of skills that are a path to self-esteem, leadership, and even jobs. It’s not an unusual youth educational model—except that almost no organizations, if any, had figured out a way to make bike-building seem cool to young people until BNB began doing so.

Located in a side alley and approached by a concrete entrance ramp decoratively bordered with chainring imprints, the Hub is equal parts warehouse, office space, mechanic bench, and teen ­drop-in.­­ Expressive ­pro-bike sentiments are sticky-stuck everywhere, including a urinal that has a small car sticker positioned for target practice. In the reception area, a stationary bike is rigged to power a smoothie-making­ blender. The walls that are not covered by heaps of ceiling-bound bikes are plastered with articles and aphorisms, and images of the staff and students on rides. Almost no one wears spandex or racing uniforms in these images; instead, it’s anything from double-breasted pea coats and ballet flats to Jordans and jeans. In the weedy backyard sprout dozens of refurbished bikes and an ungainly pile of rescues from a recent community bike drive. When a visitor asks a staff member if she worries about someone jumping the chainlink fence to liberate a 10-speed, she smiles and says, “If they jump the fence and work that hard to get a bike, maybe they just really need one.”

Teens don’t initially come through the Hub’s heavy loading-­dock doors with that sort of outlook—nor are they thinking brightly about “peer to peer modeling,” or life skills. “We kind of get them in the door with the free-bike thing,” says Sarah Braker who along with longtime EAB director Elijah Evans make up the full-time Youth Programs staff. “What happens after that is the magic part.”

In the first hour of the first day of class, students choose a bike from the hundreds of discards warehoused at the Hub, then completely strip it down to the frame. Imagine junior med students with a ­cadaver. The harm has already been done. Tools can be liberally wielded, hammers joyfully raised, long pipes employed for leverage to pry loose bolts. There is no breaking these already-broken bikes. For the students, the exercise demystifies everything: the tools, the bike parts, and each other.

What happens next is anything but demystifying. The tools are put down. Untrained mechanics stare at a naked frame. They have no idea what to do next. They’re given a sheet that documents all the hours that will be required of them to build from nothing a working bicycle they can keep. The sheet is not just a guide, but also a way to track time: One hour of work equals two credits. Earning an average road bike with 10 or more speeds or a low-cost mountain bike, requires 130 credits. Basic accessories such as racks or fenders or pegs require still more hours. The looks on the faces as the students consider the frame and their sudden heavy indenture to it ranges from disbelief to faint hostility, say the instructors. Everyone now truly understands something important: The free bike isn’t free.

The man who quickly explains to them why that isn’t a bad thing is Elijah­ ­Evans. He is a towering presence in a dark Phat Farm sweatshirt and long, shoulder-length dreadlocks. He is also a pro-level bike mechanic. Holding a dry-erase marker in front of a white board that on one side depicts illustrations of cable routing for brake and shift levers and on the other holds exhortations such as “use appropriate language!” he is Room 222 meets Sheldon Brown. At 14, a self-described “big black kid from Dorchester who was supposed to be a certain way,” he attended the course at the insistence of his single mom. He got not just a bike but a second home, a first job, and the discovery of his talent as a natural-born teacher. Now 23, he has worked at BNB for nine years and, though everyone on staff expresses the wish that he work for at least nine more, he was recently accepted to Northeastern University’s School of Law.

As he looks out on the class I visit, he sees even this late into the course—though he would never put it this way—one or two dark horses, a student or youth instructor who can’t find their footing. A year ago, one of the worst was so disruptive and so plainly disinterested that for the good of the rest of the class some of the staff recommended expulsion. Evans counseled patience, the youth stayed, and by graduation was one of the class leaders—and later returned to work as a wage-earning teen instructor.

“You know at this point if you’re asked by anybody to fix a part of your bike you should be able to do it,” he tells the students. He hopes they understand that this reminder is not just about their bikes.

Two days before graduation, Lucie—small, blonde, big of personality—still isn’t sure about her bike. She chose the kind of touring frame she’d seen her parents’ friends riding, and now has the green Fuji’s fork in one hand and with the other is rummaging in a cascading parts bin for a straight handlebar to replace the drops. It won’t be the most rideable setup, plus it’s another $10 upgrade (five more working hours). Other than asking, “you sure you want to do this?” none of the instructors genially swarming the area interferes. They understand that Lucie needs to find her own way. They’ve each made their own bike (or, like 16-year-old Abdul, eight). They also possess an unsettling comprehension of what to do with adults, like me, who want to help disadvantaged youth. Part of this comes from building the bikes, part from the training they must take to become instructors, and part from training adults who want to volunteer as part-time instructors. Ronald, a high-school junior originally from the Dominican Republic,­ advises a ­Boston lawyer what to do when attitudinal issues arise. Stephane, a Boston Latin Academy junior who never pedaled a bike more than 15 miles until last spring, instructs a racer from Canton on how to guide a ride. “It was awkward at first,” she says in the same tone she might admit that the once-bewildering challenge of removing a pedal is now second-nature.

The institution’s absolute trust that the youth can and will come through with sound decisions can be disorienting to newcomers and outsiders like me. This is never more apparent than during one of the program’s signature events, the ­closing week’s Commuter Challenge. The class is split into three teams, heading from the Hub to a common downtown destination by car, subway, and bike.

I join Mohamed and his fellow youth bike guides as they prepare to supervise five young riders and two adults in navigating 3 miles’ worth of rush-hour city streets to a Kenmore Square Best Buy store. ­Mohamed comes out wearing a black, full-face downhiller’s helmet.

I’ve been warned by Evans and others not to be “ageist.” Braker has reminded me that a contingent of BNB teen employees had, on their own, recently applied for and been awarded a $1,000 grant to bring youth-run mobile bike shops to the poorest parts of the city. Arik Grier, another longtime staffer, who seemed to anticipate that I would experience a ­moment of deep doubt on the ride, tells me that someone like myself might experience a kind of enlightenment once I get over my power hang-ups. During the ride out to Best Buy, I congratulate myself for being that transcendent guy as I witness the group being all “rider on your right” and expertly escorting one another across raging Boston roadways.

But enlightenment is never that easy. On the return home, when things finally get tough, I’m back to being ageist. Morale isn’t good. The cycling team got beat to the Best Buy by the group that took the T, Boston’s subway system. Plus it’s much colder than it was an hour ago. Plus it’s dark. The singsong “rider on your right” and “taking the lane” courtesies have stopped. I’m suddenly, and acutely, aware that in this traffic all of our lives depend on those who, when there’s a break for some free time at the Hub, like to launch down the stairwell on mini bikes.

Yet our dicey return, which features falls, flats, and a few tears, ultimately works out because of the unflappable youth leading it. They modulate our speed to accommodate everyone, they take turns pausing in intersections like school crossing guards until the whole group passes safely through—an advanced group-commuting technique known as corking traffic—and they comfort Annie, who becomes increasingly pissed she ended up with a 21-speed bike on which 20 don’t work. Mohamed quietly coaxes her through the darkened bike paths, across streets, and, even more amazingly, into a starring role in the debate back at the Hub that ends the challenge.

As the groups argue over who won the challenge—based not just on who reached the store first—Annie proclaims that mood-enhancing “happy ­hormones” produced by riding a bike create a health benefit the other forms of transportation lack. Though there is debate about her claim—“you didn’t look too happy when you got in here a few minutes ago,” ribs Lucie—Evans awards the winning points to Annie’s team.

“It’s a fact,” Evans says,” that when you exercise you produce endorphins that make you feel better.” Annie looks triumphantly to a still somewhat stunned Mohamed, and says, “I told you so, didn’t I? I knew that in like fourth grade!”

It’s a memorable, oddly uplifting day for many reasons. On our ride home, Mohamed had asked me to do the final bit of traffic control at the corner of Green and Amory. As he gave the nod to a 50-year-old corking newbie, telling me to go ahead and see what it feels like to do what I’m told no matter what I think and stand in dark Boston traffic in service to others, I had the distinct impression that the teacher had sensed the moment for true enlightenment.

Kelly is the first to have her bike ready for the graduation ceremony. She is also the first person in her family to know how to fix a flat and repack a hub. As I continue to try to understand what draws these teens to such an unglamorous undertaking when so many of their peers are home watching reality TV, Kelly makes me realize that being able to do something nobody else in your family can, including your dad, isn’t a bad start.

The 12 students in this class, an almost equal number of youth instructors, a supervising mechanic, and Evans twist and tweak on the bikes almost until the moment family and friends start arriving at the alleyway entrance. This is crunch time, and there’s a little steel in Evans’s voice as he implores his staff to be extra vigilant about final checks. He’s worried about the basics such as loose locknuts, twisted stems, and bad brakes. But he’s also got Roshawn getting a blowtorch white hot so he can try to free somebody’s frozen seat tube. Kenny is concentrating on an R&B playlist for the ceremony. Jose needs to overhaul a free wheel. For his first time. The air crackles like an ER on a Friday night.

When the show starts, not a thing goes wrong. First there are demonstrations of what this new generation of wrenches has learned. Deirdre and her twin sister from Mattapan command crowds two and three deep as they repack a hub. At another station, Lucie and Pretzel pass out the class-made cookbook focusing on unadorned, nutritionally power-packed items. Even Allen Lim would be ­hard-pressed to argue with the simple brilliance of page one’s “boiled eggs.”

As well as all this goes, it’s mere prelude to the show-stopping part of the evening when the students are individually introduced, awarded graduation ribbons (with chainring medals), and allowed to roll out their bikes in public for the first time. Chairs are rearranged to create a runway; music is cued, and to mad ­applause the parade begins. There are BMX bikes, cruisers, and vintage three-speeds. At first there is a little self-consciousness with all eyes on each graduate and their works. But by the time Lucie rolls out her bike—flat bar and all—she is full of unabashed enthusiasm, pounding high fives with everyone along her route. The final punctuation is a “whoop-whoop” that might’ve been heard at the Stony Brook Orange Line stop. For the next 10 minutes, there is bedlam. A dark, cold Tuesday in November might be the best day of the whole year of cycling in this city.

Both my children and two of their friends—who in my enthusiasm I entreatied into accompanying me—will tell me afterward that they can’t believe how cool it was. My son and daughter will later ­enroll in their school’s bike advocacy elective, and one their friends, Amadu, will raise $1,400 for charity at his private school with—what else?—a tweed ride. I’m not unaffected, either. I will begin by picking up the garage from last summer, then figuring out how to overhaul the brakes on my town bike and getting my brother to join me in a membership at the local wrench collective. In the months to come, I will end up going deeper still. I’ll help start up a local bike committee. I’ll volunteer to teach a three-week course at the local private school and, at the close of school, the students will hold a bike drive, collecting 120 bikes to donate to BNB. A few family members will ask if I might have stumbled into a new career, since, they say, I seem to have such a enthusiasm for spreading the gospel of bikes.

But for right now, amid riotous family, Fatima is giving out and receiving hugs. It is her first graduation. We all want to earn a bike.